1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method and an image processing program for performing image correction of gradation with respect to an image.
2. Related Art
In an image correction or modification technology photographed by a camera such as commercially-sold digital still camera and wide dynamic range (having expressive power more than 8 bit per each color of RGB) camera, many attentions have been paid on a technology for improving brightness such as luminance, lightness or like. In order to improve such deviation as luminance, correction utilizing single brightness character is mainly performed, and as a function for correcting the brightness character, a gamma (γ) correction function, a logarithmic function, a function defined by using statistical processing such as a histogram averaging or like has been utilized.
Furthermore, in accordance with popularization of broadband internet connection of such as optical fiber or high resolution of moving pictures in a high definition television broadcasting, machineries such as digital still cameras or digital video cameras have been highly qualified from several mega pixel to giga pixel, and according to such tendency, it has been required for an image to be processed or treated at a high speed.
In order to achieve such requirements, it is required to properly stress image contrast at high speed, and for example, Patent Publication 1 (Japanese Patent Publication No. 4214457) discloses a following image processing device for obtaining gradation compressed images. In this image processing device, luminance of an image of wide dynamic range is subjected to logarithmic conversion. In a next step, logarithmic tone curve correction image is produced. Subsequently, a logarithmically reduced image is produced, and thereafter, luminance of a logarithmically smoothed image is calculated by an interpolation operation.
In a further next step, gain values are calculated with respect to respective pixel positions. Thereafter, the luminance of the pixel of the logarithmically gradation compressed image is calculated, and the luminance is subjected to logarithmically reverse conversion.
However, when a whole image is divided into blocks and the blocks are then processed respectively for highly speed-up of the image processing, block noise is generated. In addition, in a conventional image correction technology, when blacked portions in an image are corrected, unnatural portions may be produced in a contour portion of the image or an area in which specified luminance is concentrated. Thus, it may be said that the conventional technology was not sufficient.